I just took a look at the Auctions, and there's a 94 Durant on sale for an opening bid of 305,000 coins, or 325,000 to buy outright. Of course, since it's not regulated, most people try to grossly overcharge for cards like that. There's always a Shaq or Wilt on there for 1,000,000 coins.

So you could try your luck with a price in the same range as that Durant, or if you want to actually sell it, you could probably go a bit lower. In lieu of regulation/upper limits on card prices, Ultimate Team ideally needs a feature similar to eBay where you can see what price certain cards sold for, to give you an idea of the value, and what kind of price people are actually willing to pay.